Joellen Russell wasn’t prepared for the 10-metre waves that pounded her research vessel during an expedition south of New Zealand. “It felt like the ship would be crushed each time we rolled into a mountain of water,” recalls Russell, an ocean modeller at the University of Arizona in Tucson. At one point, she was nearly carried overboard by a rogue wave.

But what really startled her was the stream of data from sensors analysing the seawater. As the ship pitched and groaned, she realized that the ocean surface was low in oxygen, high in carbon and extremely acidic—surprising signs that nutrient-rich water typically found in the deep sea had reached the surface. As it turned out, Russell was riding waves of ancient water that had not been exposed to the atmosphere for centuries.

Although controversial when she encountered it back in 1994, this powerful upwelling is now recognized as a hallmark of the Southern Ocean, a mysterious beast that swirls around Antarctica, driven by the world’s strongest sustained winds. The Southern Ocean absorbs copious amounts of carbon dioxide and heat from the atmosphere, which has slowed the rate of global warming. And its powerful currents drive much of the global ocean circulation.